In recent years, there has been a great deal of interest in the transmission of video signals via optical fibers. This mode of signal transmission offers a number of advantages over transmitting signals by conventional coaxial cable, the manner in which video signal distribution is now commonly accomplished in CATV systems. Optical fibers intrinsically have more information-carrying capacity than do coaxial cables. In addition, there is less signal attenuation in optical fibers than in coaxial cables adapted to carry radio frequency (RF) signals. Consequently, optical fibers can span longer distances between signal regenerators than is possible with coaxial cable. The dielectric nature of optical fibers eliminates any problems with electrical shorting and they are immune to ambient electromagnetic interference (EMI) and generate no EMI of their own.
The amplitude (intensity) modulation of an optical signal with a broadband RF signal requires a light modulating device, such as a laser, which has linear characteristics over a wide dynamic range of operation. Until recently it has been difficult to fabricate lasers in which the relationship between the input current and the optical output was linear over more than an extremely limited range. Because of this difficulty in obtaining lasers which were sufficiently linear to support analog amplitude modulation, digital modulation was, until recently, the primary means for transmitting information by optical signals. Digital modulation does not require a laser with a large dynamic range as do analog means for transmitting information, e.g., amplitude modulation or frequency modulation of a carrier frequency modulating the laser output.
However, because of the broadband nature of video signals, digitization of these signals consumes extremely large amounts of channel capacity. A typical analog video signal occupies 6 MHz of bandwidth. Transmission of this information digitally requires a digital data transmission rate of approximately 45 Mb/sec. High definition video (HDTV) may require a digital data transmission rate of up to 145 Mb/sec. Moreover, encoders and decoders for converting analog video signals to a digital form and for reconverting these digital signals to an analog form for viewing on a conventional television set are quite expensive. Consequently, the analog transmission of video signals is potentially much more economical than digital transmission of such signals.
Recent advances in laser technology have made the analog modulation of optical signals more feasible. Currently available Fabry-Perot (FP) and Distributed Feedback (DFB) lasers have sufficiently linear characteristics to allow them to be used as analog modulators of optical signals.
One such means of analog transmission is to use a baseband television signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is, in turn, used to modulate an optical signal. Such frequency modulation is less susceptible to noise than is amplitude modulation, but it requires more bandwidth for each television channel transmitted than is required by amplitude modulation methods. Thus, the number of television channels which can be carried by each optical transmission on separate optical fibers in an FM-based system is somewhat limited. Moreover, since the standard NTSC format for video calls for amplitude modulation of the video carrier, means for converting FM signals to an NTSC amplitude modulated format are required either at the television receiver or at the point at which the fiber transmission trunk is connected to a coaxial cable distribution network. The need for such FM to AM conversion increases the cost of the system.
In view of the above, a system in which a video baseband signal amplitude modulates a radio frequency carrier signal, which in turn intensity modulates an optical signal, is preferable to other systems from the standpoint of cost and simplicity. However, several phenomena limit the number of radio frequency channels which can be carried by present day optical links where the intensity of the optical signals is amplitude modulated. The first of these phenomena is a limitation of the amount of radio frequency energy which may be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device.
This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it is desired to transmit 80 radio frequency channels over a single optical link, the power available for each channel is only half of the power which would be available if only 40 channels were transmitted by the optical link. Such a limitation on the power of each radio frequency carrier brings each of these carriers closer to the "white noise" level of the system, thus, adversely affects the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers which must be used and the overall complexity and cost of the system.
In addition, increasing the amount of radio frequency power supplied to the laser beyond certain limits may cause the laser to produce several types of distortion. When the modulating signal supplied to a laser causes the laser to be driven into a nonlinear portion of its input-signal-to-light-output characteristic, harmonic distortion may be produced. The products of this type of distortion are signals which are integer multiples of the "primary" frequency or carrier frequencies of the video signals. The second harmonic of 54 MHz is, for example, 108 MHz. Thus, if the bandwidth accommodated by a system is such that there are channels at both 54 MHz and 108 MHz, second harmonics of the 54 MHz channel will interfere with the signals on the 108 MHz channel.
Intermodulation distortion is also of particular concern in amplitude modulated systems. Such distortion results in distortion products at frequencies which are the sum or difference of two other frequencies. The distortion products which are the sum or difference of two primary frequencies are termed second order distortion products and are particularly troublesome. For example, a video channel at 150 MHz and another video channel at 204 MHz may produce a second order distortion product at 54 MHz (the difference frequency) and at 354 MHz (the sum frequency). Significant third order distortion products may be generated by mixing signals at three frequencies or by third harmonic generation. Lesser, third order distortion products are produced by the mixing of a primary frequency with a second order distortion product. This produces third order distortion products at the sum and difference between the primary frequency and the frequency of the second order distortion product.
Clearly, one method of dealing with the above problems is to utilize detectors and optical receivers which are highly linear and which are relatively insusceptible to harmonic and intermodulation distortion. It is especially important that the production of second order distortion products be minimized. Optical receivers are combinations of optical detectors and specialized receiver amplifiers which serve to convert the amplitude modulated light to conventional broadband RF output signals comprising multi-channel video and/or data signals. Such optical receivers should be effective over a bandwidth of approximately 50 MHz to 550 MHz, or higher, so as to be compatible with current coaxial cable transmission technology. It is presently desirable that an optical receiver be effective at frequencies greater than 550 MHz in order to accommodate additional bandwidth which may be required in future CATV systems.
Optical detectors for converting the amplitude modulation of an optical signal to a radio frequency electrical signal corresponding to the modulation may comprise, for example, photodiodes. This type of device produces an output current corresponding to the amplitude or intensity of light applied to it. One advantageous type of receiver amplifier which has been used for converting the output current signal from such a photodiode to an RF voltage signal suitable for transmission on a conventional coaxial cable for broadband signals is known as a transimpedance amplifier.
The transimpedance amplifier tends to avoid the high frequency roll-off problem associated with other high impedance amplifiers. The transimpedance amplifier is similar to other high impedance amplifiers except for the addition of a feedback path, for example, between the drain and gate of a field effect transistor in a common drain configuration. A characteristic of this circuit is that its input impedance is approximately equal to the feedback impedance R.sub.f divided by 1 plus the transconductance G.sub.m of the circuit or (R.sub.f /1+G.sub.m). Thus, depending on the selection of an appropriate value for R.sub.f, the input impedance of a transimpedance amplifier can be lower than that of a similar high frequency amplifier. This relatively lower input impedance minimizes the problem of high frequency roll-off in the 50 to 550 MHz frequency band.
Transimpedance and other high frequency amplifiers are both susceptible to second order and other even and odd order distortion problems when they are used for the amplification of broadband signals which include a high number of video carrier frequencies. In high impedance amplifiers, these distortion products tend to be more severe at the low end of the frequency band. In transimpedance amplifiers, the problem of second order distortion products is essentially the same throughout the band of operation.
The troublesome second order distortion problems can be alleviated in transimpedance amplifiers by using a dual matched amplifier configuration coupled in a push-pull relationship. Since the nonlinearities of each of the amplifiers can be made to be relatively similar, the balanced configuration of a push-pull coupling tends to cancel out these nonlinearities and thus alleviate the problem of second order intermodulation products of the input frequencies being produced. An advantageous push-pull transimpedance amplifier for an optical receiver is disclosed in a U.S. patent application Ser. No. 481,436 now U.S. Pat. No. 5,239,402 entitled "Push-Pull (optical receiver)" filed on Feb. 16, 1990 in the name of Little, et al.
Optical receivers have previously used transimpedance amplifiers which have two stages. In general, such amplifiers include a transimpedance stage comprising a field effect transistor or other amplification device in a common source (emitter) configuration. The output of the transimpedance stage is then buffered by a second stage used for impedance matching and power transfer to a output power combiner.
While extremely advantageous for linearizing and converting broadband optical signals into RF signals, this configuration does present some problems. An initial problem is inherent in the common drain (emitter) configuration and is termed the "Miller effect" where the amplifier stage amplifies the feedback capacitance of the device by its voltage gain and thereby reduces the high frequency response of the amplifier. Because most devices configured for voltage gains produce such effects, it is difficult to increase the bandwidth of the amplifier.
In high sensitivity systems, such as broadband optical receivers, noise performance is critical. Unfortunately, there is always some noise associated with the amplifier itself and, in the present configuration, some noise can also be attributed to the buffer stage. The choice of the configuration for the output impedance matching stage to minimize this additional noise is important.
Another inherent difficulty with this configuration is, because there are two cascade stages, there are two bias currents even when the optical receiver is in a quiescent state. This limits the efficiency of the conversion process where output power is limited to some percentage of input power. Moreover, when there are two bias circuits in each amplifier, the exact matching of both amplifiers for distortion cancellation in a push-pull arrangement becomes more difficult.